1. Field of the Invention
This invention relates to a laser device monolithically integrated with an MQW (Multi-Quantum-Well) modulator. More specifically, this invention concerns the specific structure of the laser device, wherein the oscillating frequency or phase thereof can be controlled by a bias voltage applied to the MQW layer.
2. Description of the Prior Art
In recent years, many light communication systems have been installed using a semiconductor laser as a source of coherent light modulated in a digital or pulse signal. An injection current to the laser is switched from one state to another state by outside circuitry. At present, pulse modulation is the most popular technology in light communication systems. However, modulations, such as FM, PM, FSK and PSK, now widely used in other types of electrical communication systems, are desirable. These modulations, however, involve difficult problems when applied to light communication systems. In order to transmit a lot of information or to increase a transmission distance without amplification, it is desired to utilize the modulation method other than pulse modulation. To achieve this object, a laser device or an apparatus must be developed such that the oscillating frequency or phase (these words depend on the modulation method, therefore, hereinafter "light wavelength" is used instead of frequency or phase to simplify the explanation) can be controlled by an electrical modulating signal from outside circuitry.
The light wavelength of a semiconductor laser is determined primarily by factors such as, band-gap of a semiconductor medium constituting the active layer of the laser, a longitudinal length of the active layer defining a resonance of oscillation, for a DFB (Distributed Feed Back) laser, a pitch of diffraction grating, an effective refractive index of a light resonator in the laser, an operating temperature thereof, etc. However, the length and the material used in a laser can not be changed once it is fabricated and put into operation.
In designing a laser device, the semiconductor material of an active layer is first considered. A semiconductor has its own band-gap characteristic, and the band-gap principally determines the oscillating light wavelength. However, the light output versus wavelength relation of a laser shows a curve having a peak value at the center wavelength and gradually drooping on both sides thereof. The actual oscillation wavelength is further controlled by dimensional factors regulating the light resonator, a plasma state of the semiconductor material in an active layer, an injection current, a refractive index, etc. Moreover, the refractive index also changes depending upon the plasma state of the semiconductor. The structure of the laser is designed to take all these factors into consideration.
As described above, the refractive index can be changed by controlling the injection current to the active layer of the laser. However, it is difficult to utilize this phenomenon directly to control the oscillation wavelength by modulating the injection current, because the control of the current flowing across the active layer in a forward direction causes some problems such as a leakage current or a temperature rise of the laser device during operation.
In one solution of the prior art, the laser is separated into two parts, arranged on the same substrate, by a crossing trench, which is vertical to a light propagating direction. The first part is utilized for laser oscillation, and the second part is utilized for changing the wavelength by controlling the injection current thereto. This results in changing the plasma state of the second part which changes the composite value of the overall refractive index.
Another method of controlling the wavelength is to utilize a reflecting mirror or diffraction grating disposed outside of the laser. This results in changing the wavelength of the composite resonator. However, it is difficult to tune the wavelength electrically.
These methods of the prior art for controlling the wavelength have problems such as a variable range of 50 .ANG. is achievable at most for 1.3 to 1.6 .mu.m wavelength range, and the light output is unstable.